This invention relates to methods and apparatus for joining metallic surfaces and is particularly concerned with a solder preform and methods employing the same for providing a solder joint between two solderable surfaces.
The invention is particularly useful, for example, in the manufacture of electronic devices employing high power dissipation semiconductor components, such as power dissipation semiconductor components, such as power transistors and the like. In the assembly of such devices, a semiconductor chip with a metalized undersurface, generally referred to as a die, is soldered to a metal surface of a support structure generally referred to as a header. The header structure functions as a heat sink through which heat is transmitted away from the die to prevent thermal overload of the semiconductor component. Typically, an electrical connection is made to the die via the solder joint with the header. For example, in the case of a power transistor, a connection to the collector will often be made through the header, while connections to the base and emitter are made through wires bonded to metalized portions of a top surface of the chip.
In order for such devices to operate continuously at high power levels, the solder joint between the die and the header must exhibit good thermal and electrical conductivity uniformly along the interface between the die and header. In addition, for the device to be practical, the solder joint must be strong enough to withstand structural stresses which might be experienced during use. To achieve the foregoing characteristics, the solder joint must be kept to a minimum thickness and must be substantially free of voids between the bonded surfaces of the die and header.
Perhaps the most basic technique for soldering a die in place involves the use of a solder body or preform of closed geometric shape (e.g., rectangular or circular solid) which is placed upon a support surface of the header, with the die being placed upon the preform. These components are then passed through a controlled atmosphere furnace at a temperature above the solder melting point but below the melting point of the surfaces to be bonded. The furnace atmosphere is usually hydrogen or a hydrogen-nitrogen mix (known as forming gas) which inhibits oxidation of the surfaces to be bonded and of the preform. On passing through the furnace, the solder preform melts and wets the metalized surface of the die and the header support surface. The assembly is then cooled so the solder solidifies to effect a bond.
In practice of the foregoing technique, the formation of voids in the solder joint can be particularly problematic. Void formation is generally the result of gases which are entrapped between the solder preform and the opposing surfaces of the die and header during formation of the solder joint--that is, when the solder is melted. If the voids are not dispersed over the bonded surfaces of the die and header or cover an excessive area (e.g., 10% or more) of the bonded die surface, or if a single void is too large (typically, in excess of 5% of the bonded surface area of the die), thermal failure of the semiconductor component can result.
Another problem with the previously described soldering technique arises from the extreme thinness of the preform. Because the preform must be made very thin to control the amount of solder in the bond, uneven melting of the preform can remove support from certain areas under the die and cause shifting of the die on the header due to surface tension of the melting solder. This shifting can make subsequent bonding of wires to the die difficult due to poor alignment of the die and is therefore highly undesirable. Also, the thinness of the preform can cause difficulties in handling.
One technique which has been proposed for providing a better solder joint in the manufacture of semiconductor devices is disclosed in U.S. Pat. No. 3,786,556 issued Jan. 22, 1974 to Weston. Weston's technique involves the placement of preferably two preforms laterally adjacent the die on the support surface of the header. Upon passage of the header with the preform and die thereon through a furnace, the preform melts and solder flows between the opposed die and header surfaces by capillary action to fill the area between the surfaces. The flow of solder toward and between the surfaces may assist in the expulsion of gases from between the surfaces. As a modification to the foregoing technique, Weston also proposed the placement of two solder preforms at spaced locations on the support surface of the header and placement of the die on the preforms, with peripheral portions of the preforms supporting peripheral portions of the die. The flow of solder laterally into the space between the preforms is said to expel gases from between the surfaces to be bonded.
While techniques involving two spaced preforms may offer the advantage of reduced void formation, there are significant practical disadvantages to such techniques. First, the use of two preforms requires a significant amount of assembly time in preparation for heating. In particular, the preforms must be very accurately positioned on the header support surface prior to the placement of the semiconductor die. This is a very time consuming and thus costly operation. In addition, when such preforms are disposed on the header largely or entirely laterally adjacent the die, as discussed above, it can be difficult to control precisely the amount of solder which ultimately forms the bond since solder is permitted to flow and remain well beyond the die perimeter. Furthermore, laterally disposed preforms can also cause problems with the die shifting. The latter two of the aforementioned disadvantages are, of course, a concern even with only a single laterally disposed preform. Finally, as has heretofore been true with respect to preforms generally, the preforms of such proposed improvements are characteristically quite thin and difficult to handle.